The direct conversion of radioisotope beta (electron) emissions into usable electrical power via beta emissions directly impinging on a semiconductor junction device was first proposed in the 1950's. Incident beta particles absorbed in a semiconductor create electron-hole-pairs (EHPs) which are accelerated by the built-in field to device terminals, and result in a current supplied to a load resistor. These devices are known as Direct Conversion Semiconductor Devices, Beta Cells, Betavoltaic Devices, Betavoltaic Batteries, Isotope Batteries etc. These direct conversion devices promise to deliver consistent long-term battery power for years and even decades. For this reason, many attempts have been made to commercialize such a device. However, in the hopes of achieving reasonable power levels, the radioisotope of choice often emitted unsafe amounts of high energy radiation that would either quickly degrade semiconductor device properties within the betavoltaic battery or the surrounding electronic devices powered by the battery. The radiated energy may also be harmful to operators in the vicinity of the battery.
As a result of these disadvantages and in an effort to gain approval from nuclear regulatory agencies for these types of batteries, the choice for radioisotopes has been limited to low-energy beta (electron) emitting radioisotopes, such as nickel-63, promethium-147 or tritium. Due to the fact that promethium-147 is regulated more stringently, requires considerable shielding, and nickel-63 has a relatively low beta flux, tritium has emerged as a leading candidate for such a battery device.
Tritium betavoltaic batteries, sometimes referred to as tritium betavoltaic devices or tritium direct conversion devices, have been promoted during the last thirty years. Tritium is a relatively benign radioisotope with low beta energy emission that can easily be shielded with as little as a thin sheet of paper. Tritium has a long track record in commercial use in illumination devices such as EXIT signs in commercial aircraft, stores, school buildings and theatres. It is also widely used in gun sights and watch dials, making it an ideal power source for the direct conversion devices. Given the low power and relatively large size of a typical tritium betavoltaic cell, it has been difficult to produce a device with meaningful power that is both cost-effective and space-efficient.
Several attempts have been made to produce useful current from a tritium betavoltaic battery. For example, polycrystalline or amorphous semiconductor devices have been considered for tritium betavoltaic batteries based on the assumption that such devices would allow batteries to be fabricated at a reduced cost. It is assumed that these devices could be manufactured in a thin-film like fashion and that tritium could be embedded within the polycrystalline or amorphous devices. However, this approach is extremely inefficient (much less than 1%) with respect to the beta energy emissions entering the semiconductor. The main reason for this low semiconductor conversion efficiency is the high dark current or leakage current of the semiconductor that acts as a negative current. This high dark current competes with the betavoltaic current produced by collection of EHPs created via the tritium beta particles impinging on the semiconductor. In short, the polycrystalline and amorphous semiconductors have a high number of defects resulting in recombination centers for the EHPs, which in turn significantly reduce the betavoltaic current and lead to very low efficiency for the battery.
The best results for tritium betavoltaics have been achieved with single crystal semiconductor devices. Recent attempts have involved single crystalline semiconductor devices with a tritium source such as a tritiated polymer, aerogel or tritiated metal hydride placed in direct contact with a semiconductor junction device. Single crystalline semiconductors have longer carrier lifetimes and fewer defects resulting in much lower dark currents. Though successful use of single-crystal betavoltaics has improved the power yielded from a single betavoltaic cell, reductions in cost and the volume utilized for a single cell needs to decrease to allow betavoltaics to compete in the market place.
In addition to the above listed obstacles, the texturing of a direct conversion semiconductor device for the purpose of increasing the surface area exposed to beta radiation emission has been proposed several times in the past. For example, on page 282 of the book entitled “Polymers, Phosphors and Voltaics for Radioisotope Microbatteries” edited by K. Bower et al., the use of porous silicon and tritium inserted into porous silicon holes was proposed as a means of increasing the surface area of the semiconductor device by 20 to 50 times, in contrast to the original planar semiconductor surface area.
Each of the following published patent applications and patents propose a method for increasing the surface area of the semiconductor by textured growth of the semiconductor or a post-growth texturing method:                US Patent Application Publication 2004/0154656        US Patent Application Publication 2007/0080605        U.S. Pat. No. 7,250,323        U.S. Pat. No. 6,949,865        U.S. Pat. No. 7,939,986        U.S. Pat. No. 7,663,288        US Patent Application Publication 2011/0079791        US Patent Application Publication 2007/0080605        
Central to the approaches of the above-listed patents and published patent applications is a belief that an increase in surface area exposed to radioisotope emissions will increase the power per unit volume of the direct conversion semiconductor device. The overall goal of this approach is to not only reduce the size of the direct conversion device but also to potentially reduce the cost associated with producing the equivalent surface area in a planar semiconductor device.
The problem with such an approach arises from several competing factors. The incident power from candidate radioisotopes for betavoltaics (e.g. tritium, promethium-147, nickel-63) is quite small per unit area exposed, the dark current of the semiconductor device is a very significant factor in the overall efficiency of the device; this is especially problematic when tritium is utilized. If the dark current of a device is high, due to recombination or trapping defects in the semiconductor, then the efficiency will be especially low. For this reason, it is preferable to use single crystal semiconductors that maintain a seed or preferred orientation where device defects are minimized and the dark current is sufficiently low so that power can be produced efficiently.
Unfortunately, alterations to the semiconductor junction's crystal structure, as proposed in the above-listed patents and published patent applications, risk increasing lattice defects, resulting in a high number of recombination centers for EHPs. Using the conventional processes (e.g. surface modification of single crystal betavoltaic junctions via etching/micromachining techniques in the above patents and published patent applications) typically results in creation of a direct conversion semiconductor device with a low open circuit voltage and reduced short circuit current resulting in a low overall efficiency. In addition, edge-effects associated with highly articulated surfaces contribute to generation of trapping and recombination centers leading to overall low efficiency.